The present disclosure generally relates to a power supply, and more particularly, to an LED driver circuit with adaptive controlled output voltage for driving LEDs.
As a result of continuous technological advances that have brought about remarkable performance improvements, light-emitting diodes (LEDs) are increasingly finding use in applications such as traffic lights, automobiles, general-purpose lighting, and liquid-crystal-display (LCD) backlighting. As solid state light sources, LED lighting is poised to replace existing lighting sources, such as incandescent and fluorescent lamps, in the future since LEDs do not contain mercury, exhibit fast turn-on and dimmability, have long life-times, and require low maintenance. Compared to fluorescent lamps, LEDs can be more easily dimmed either by linear dimming or pulse-width modulated (PWM) dimming.
A light-emitting diode (LED) is a semiconductor device that emits light when its p-n junction is forward biased. While the color of the emitted light primarily depends on the composition of the material used, its brightness is directly related to the current flowing through the p-n junction. Therefore, a driver circuit providing a constant current is desired.
FIG. 1 shows a lighting apparatus including a plural number of LED strings 1 to n driven by constant currents provided by current regulators 1 to n. Each current regulator receives an input voltage VIN and provides the (same or different) current through respective LED strings, each of which includes at least one LED. LED strings 1 to n may generate lights of the same color or different color depending on the color output requirements of the lighting apparatus.
A current regulator may be a linear current regulator, such as the regulators 16-1 to 16-n shown in FIG. 2 or a switching current regulator, such as the regulators 18-1 to 18-n shown in FIG. 3. A linear current regulator generally has lower cost than a switching regulator. However a linear current regulator may have lower efficiency than a switching regulator depending on the voltage drop across the regulator.
In FIGS. 2 and 3, a power source 12, which may be DC or AC, provides an input voltage for a switching pre-regulator 14, which regulates an output voltage VB. Each current regulator 16-1 to 16-n, 18-1 to 18-n, receives the voltage VB and regulates the driving current through a respective LED string. The power source 12 can, for example, be a battery or an AC source from the utility power line. The switching pre-regulator 14 can be any type of DC/DC or AC/DC converter, and can be isolated or non-isolated depending on the application requirement.